Shock chlorination is a procedure used whenever there is a need for emergency disinfection of tanks, wells and/or distribution systems where there is confirmed evidence of microbiological contamination (i.e., positive coliform or E. coli (pdf) samples). Again, this procedure is for emergency purposes only and should not to be used on a regular basis.

See the DWS Well Disinfection (pdf) technical bulletin for shock chlorinating a well and small distribution system (no storage tank).

Shock chlorination of a storage tank or reservoir consists of the following steps:

In order to appropriately utilize the Shock Chlorination Calculation tool (xls), users must enter data in the yellow-highlighted areas, and know the required residual chlorine concentration AND tank, well, or piping volume to be shock chlorinated.

If using a volume representing tanks, pipes, and wells, please apply the highest chlorine residual concentration and the longest time needed to disinfect any one of the above components.

1. Calculate Tank Water Volume

Information on tank water volume should be on file with the water system. If water volume information is not readily available, please use the storage tank dimensions (measured in feet) as the data for calculations. Included below are formulas for determining rectangular and circular-shaped tank water volume. Please use the appropriate formula.

Rectangular tanks: Water volume is length times width times height of the overflow port height (use the top, to be conservative) times 7.48 gallons per cubic foot.

Example: VolumeR = Length x Width x Height of overflow port x 7.48

Circular tanks: Water volume is the radius (i.e., half the diameter) squared times π (pi; i.e., 3.1416) times the overflow port height times 7.48 gallons per cubic foot.

2. Determine Time Frame or "Method Exposure"

There are three exposure time-frames that can be used: 3-hour, 6-hour, and 24-hour.

Permissible exposure time depends on the chlorine concentration applied and whether the chlorine can be added as the tank is being filled.

Paired time and concentration is referred to as a "Method" as per Standard C652-11 of the American Water Works Association. Exposures of longer duration are regarded as conservative and generally result in better disinfection.

3. Add the Correct Amount of Bleach

After determining the tank, well, and/or pipe water volume, you can input that value (in gallons) in the Shock Chlorination Calculation tool (xls) in the yellow-highlighted cell labeled "Volume to be disinfected =".

Entering or changing the tank volume will automatically update the values under the "Chlorine Source Material..." heading, which report the volume (gallons) or weight (pounds) of chlorine needed to achieve the target dose depending on the form of chlorine used.

The desired chlorination dose is either 10 or 50 mg/L (unless the "Method" is spray application); these chlorine concentrations are indicated in the yellow-highlighted cells directly beneath the "Method A" and "Method B" column headings. Verify the dose by analyzing for free chlorine concentration and add more bleach if necessary to achieve the Method's concentration.

4. Wait for Disinfection to Occur

In order to fully and appropriately utilize this procedure, the water system operator must wait the full duration of the "Method Exposure Time" specified in the Shock Chlorination Calculation tool (xls). Below is an example of a procedure and situation:

Question: Given that the tank storage volume is 10,000 gallons, there is no other storage for the system, and we need to get water production back online as soon as possible (thus using the 6-hour time-frame, Method B), how much normal household bleach do I need to add in order to disinfect my tank? Also, when does the "Method Exposure Time" start?

Answer: Based on the Shock Chlorination Calculation tool (xls), add 10 gallons of normal household (5%) bleach to disinfect the tank (see BLEACH WARNING, or the Technical Bulletin: Well Disinfection (pdf)). Measure the chlorine concentration. If the bleach was well mixed in the tank and the measured concentration is 45 mg/L, add about 10% of the original dose, or 1 gallon of bleach and mix. Say the measured concentration is now 54 mg/L (more than 50 mg/L), so start the clock on the 6-hour interval.

Note: Mixing can be achieved by re-circulating water in the tank. Chlorine demand is not usually so high that Method B would require 10% more dose than calculated in the Shock Chlorination Calculation tool (xls).

Warning: Bleach used in this disinfection process must be flushed thoroughly from all service lines. This procedure is for shock disinfection only, and should not to be used on a regular basis. Bleach contains chlorine and is harmful to organisms living in water and soil. Human exposure to strong bleach solutions (i.e., over 4 ppm chlorine) may cause severe irritation to eyes and skin. Bleach solutions over 4 ppm chlorine can be harmful if swallowed. Please use appropriate protection and precautions when handling bleach and provide notification to any customers who may be receiving highly chlorinated water before the system is flushed.

6. Taking Special Coliform Samples

After flushing tank volume, special coliform (pdf) (i.e., microbiological analysis) samples will need to be taken in order to determine if the above chlorination procedure was effective. For more about laboratory reporting and special coliform reporting forms, visit the Laboratory Reporting page.

The entire chlorination process above might have to be repeated if coliform bacteria (pdf) are found to be present in the special samples. Consult with DWS Technical Services staff for more information.